The present invention relates to a method of making tools with cutting edges formed of polycrystalline cubic boron nitride (PcBN) which are bonded to a body of cemented carbide or cermet.
Cutting tools having cutting edges formed of a super hard abrasive such as a cubic boron nitride (cBN)-based material are manufactured by powder metallurgical techniques and are mainly used for the machining of cast iron and hardened steel. For cast iron, a tough material with 80-100 wt % cBN is used, while for hardened steel 10-50 wt % of TiC, TiN or Ti(C,N) is usually added. This addition decreases toughness, but greatly improves the chemical stability of the material. Most often, the PcBN (polycrystalline cubic Boron Nitride) material also contains smaller amounts (typically  less than 10 wt % each with a total maximum content of all such materials being 25 wt %) of other components, e.g., Co, Ni, WC, Al, AlN and Al2O3. These are either added to the raw material powder or obtained during processing.
PcBN cutting tools are mainly produced in two different ways:
i) By high pressure/high temperature (HP/HT) sintering of a PcBN powder mixture into a solid body that is cut and ground into a finished cutting tool insert; or
ii) By HP/HT-sintering a thin layer of PcBN powder which simultaneously bonds to a substrate (usually a cemented carbide disc), from which smaller pieces (chips) are cut out. These chips are brazed onto a regular carbide tool (e.g., insert, end-mill, drill) and ground to the finished state. The tools are relatively expensive to produce due to the many steps the product must undergo before it is finished. Also, usually only one or two cutting edges per tool are available.
Through U.S. Pat. No. 5,676,496, a technique is known for producing PcBN cutting tool inserts in a more cost efficient way. This is achieved by placing a cemented carbide or cermet substrate into a container and then packing PcBN powder into appropriately placed grooves in a substrate. The container is then HP/HT-sintered so that the PcBN powder is consolidated to a fully dense body, which is simultaneously bonded to the substrate. The substrate/PcBN compound may then directly be ground to a cutting tool insert. The main advantages with this technique are:
1. The brazing step is eliminated; and
2. The number of cutting edges per insert can be increased at a limited added production cost.
Although the method described leads to extensive cost reductions per cutting edge, it has one major drawback in that the packing of a powder mixture containing PcBN into the grooves in principle must be done manually. The poor flow properties of PcBN powder in combination with the required groove geometry make automatic processing unreliable. Apart from obvious health hazards, manual packing may lead to uneven packing density and to excessive oxygen exposure of the PcBN powder. Uneven packing density makes it necessary to choose a larger groove dimension than desired to ensure that the amount of PcBN obtained is always sufficient. Careful control of the oxygen content in the PcBN powder is critical for the HP/HT sintering since excessive oxygen negatively affects the consolidation process. In principle, one would like to have a high and highly reproducible packing density and to minimize the oxygen pickup during handling and storage.
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide a method of making tools with cutting edges formed of polycrystalline cubic boron nitride (PcBN) which are bonded to a body of cemented carbide or cermet.
It is an aspect of the invention to provide a method of making a PcBN or diamond cutting tool insert comprising:
mixing PcBN powder with a liquid and a pressing agent to form a homogenous slurry of a desired composition;
forming powder agglomerates of said mixture;
pressing said agglomerates to form a body of desired dimensions and density;
removing the pressing agent from the body at a suitable temperature and atmosphere;
raising the temperature to 1000xc2x0-1350xc2x0 C. in vacuum;
solid state sintering the body at 1000-1350xc2x0 C. in vacuum for 1-90 minutes to form a body with 35-55 vol % porosity; and
treating the porous body under HP/HT conditions to form a dense body of desired shape and dimension.